FIG. 1A shows the cross-section of a Fast Recovery Epitaxial Diode (FRED) 10 according to the prior art. FRED 10 comprises a lightly-doped N− silicon epitaxial layer 12 which is formed on a highly doped N+ silicon substrate 14. A p+ doped diffusion well 16 is formed on a portion of the upper region of epitaxial layer 12. FRED 10 includes first major electrode 18 that is in surface-to-surface contact with diffusion well 16 and silicon dioxide layer 20 which surrounds and is partially in contact with the outer periphery of diffusion well 16. FRED 10 also includes second major electrode 22 which is disposed on a surface of silicon substrate 14 opposing first major electrode 18 of FRED 10.
Diffusion well 16 of FRED 10 is relatively shallow and may range between 3 μm to 6 μm for 200–600 volt devices. It has been found that 3 μm–6 μm deep diffusion well 16 provides a good tradeoff between performance and manufacturing complexity. However, devices having shallow diffusion wells do not have the capability to absorb reverse avalanche energy well.
The difference between the bulk breakdown voltage (BV), which represents the ideal breakdown voltage for a planar junction, and the actual BV for a FRED has been used to isolate the reason for the inability of FRED 10 to satisfactorily absorb reverse avalanche energy.
Referring, for example, to Table 1, FRED 10 which has a 6 μm deep diffusion well 16 can have an actual device BV that may be between 36–70 volts lower than bulk BV. It should be noted that although the thickness of the epitaxial layer 12 contributes to the difference between the bulk BV and the actual device BV, the peak electric field is found near the corners of diffusion well 16 at breakdown due to the crowding of the electric field lines. It is believed that these localized regions of high electric field, which are near the small-radius curvature of diffusion well 16, generate “hot spots” that lead to avalanche failure.
Standard Epi Profile
TABLE 1Epi thickEpi res.JunctionDeviceOne-d BVDelta[μm](Ohm-cm)Depth (Xj)BV[V](“bulk” BV) [V]BV [V]3012634438339341263564267030146350386363414637744265
It has also been found that FRED 10 having a linearly graded or double-profiled epitaxial layer 12 still has a device BV that is lower than an ideal bulk BV.
Referring to Table 2, for example, FRED 10 having a linearly graded epitaxial layer 12, and a 6 μm diffusion well 16 exhibits an actual device BV that is between 25–32V lower than the ideal bulk BV. Epitaxial layer 12 of FRED 10 of Table 2 has a linearly graded tail (Epi 2) with a concentration of dopants that is varied during the epitaxial growth and is kept constant during the final growth of the second layer (Epi 1). FIG. 1B shows a linearly graded epitaxial layer 12 graphically.
Graded Profile
TABLE 2Epi 1Epi 2One-d BVthickEpi 1 ResThickEpi 2 ResXjDevice(“bulk”Delta[μm](Ohm-cm)[μm](Ohm-cm)(μm)BV [V]BV) [V]BV [V]15201520-163003252515201520-463523843215201520-6636539227
Referring to Table 3, as yet another example, FRED 10 having a double-profiled epitaxial layer 12, and a 6 μm diffusion well 16 can have an actual device BV which is 27–28 volts less than the ideal bulk BV. Epitaxial layer 12 of FRED 10 of Table 3 has a first layer (Epi 2) of constant concentration and a second layer (Epi 1) of constant concentration. FIG. 1C shows a double-profiled epitaxial layer graphically.
Double Profile
TABLE 3Epi 1Epi 2One-d BVthickEpi 1 ResThickEpi 2 ResXjDevice(“bulk”Delta[μm](Ohm-cm)[μm](Ohm-cm)(μm)BV [V]BV) [V]BV [V]1520153.56315343281520158635438127
Comparison of the data in Table 1, Table 2 and Table 3 indicates that by grading the profile of epitaxial layer 12, the difference between actual device BV and the ideal bulk BV can be reduced. However, the difference between the actual and the ideal breakdown voltages remains high for FRED 10 having a shallow 6μm diffusion well. Moreover, the crowding of the electric field lines near the corners of diffusion well 16 is still observed in FRED 10 of Table 2 and Table 3. Thus, profile grading does not appear to strengthen the ability of FRED 10 to absorb the reverse avalanche energy.
Referring now to Table 4, diffusion well 16 of FRED 10 of Table 2 having a linearly graded profile was extended from 6 μm to 10 μm. In order to achieve a total bulk thickness of 30 μm, the Epi layer was thickened by 4 μm. As shown in Table 4, the increase in the depth of diffusion well 16 by 4 μm reduced the difference between the actual device BV and the ideal bulk BV.
Graded Profile
TABLE 4Epi 1Epi 2One-d BVthickEpi 1 ResThickEpi 2 ResXjDevice(“bulk”Delta[μm](Ohm-cm)[μm](Ohm-cm)(μm)BV [V]BV) [V]BV [V]19201520-1103653983219201520-4103213401919201520-61026527712
Further increases in the depth of diffusion well 16 from 15 μm to 20 μm in the epitaxial layer 12 of the device of Table 4 showed further reduction in the difference between actual device BV and ideal bulk BV. While this reduction between actual and ideal breakdown voltages is partly due to the thinning of the bulk thickness caused by the deepening of diffusion well 16, the deepening of diffusion well 16 has a substantial reducing effect on the difference between the actual and the ideal breakdown voltages. This reduction is believed to be due to the relaxation of the electric field lines as the radius of curvature near the corners of diffusion well 16 is increased, as well as, the spreading of the field lines toward the main portion of the PN junction (the junction between the diffusion well 16 and epitaxial layer 12), which helps to distribute the reverse avalanche energy over a wider area.
FIG. 2 shows FRED 24 having a 20 μm deep diffusion well 16 and a graded epitaxial layer 12 of very low doping level (approximately 1×1014 cm3). FRED 24 has an actual device BV which is only about 2.9 volts less than the ideal Bulk BV at 25° C. and about only 8.3 volts at 125° C. for 100 μA. Depending on the doping of epitaxial layer 12, FRED 10 (FIG. 1A) can have an avalanche voltage that changes by 25–40 volts when the temperature of the PN junction is raised from 25° C. to 125° C. It should be noted that corners of diffusion well 16 of FRED 24 are flatter and thus have a larger radius, which, it is believed, contribute to the capability of FRED 24 to absorb the reverse avalanche energy and increase the actual device BV of FRED 24.
To obtain a deep diffusion well 16, such as the one shown in FIG. 2, diffusion of dopants must be conducted at relatively high temperatures which may be in the order of 1250° C. or higher, and typically for a long drive-in time. In contrast, shallower diffusion well 16, such as the one shown by FIG. 1A may be obtained at considerably lower temperatures, which may be in the order of about 1100° C., and for a shorter drive-in time. Given that many fabrication laboratories do not have the capability for deep diffusion at high temperatures, it is desirable to have an alternative device, which does not require a high temperature diffusion step, that is capable of absorbing the reverse avalanche energy of a FRED having a deep diffusion well 16, such as FRED 24 of FIG. 2.